Iron Sulfide Minerals Research Articles

Observations on mineral transformations and potential environmental consequences during the oxidation of iron sulphide‐rich materials in incubation experiments

SummaryThis paper attempts to acquire a good understanding of the formation and evolution of acid sulphate soils (ASS), as well as to assess the consequent environmental impacts. An incubation experiment to simulate the development of ASS under various weathering scenarios was set up. Fresh monosulphidic black ooze (MBO) material was divided into three parts and each was subjected to different drying and rewetting regimes by controlling an artificial water table in an incubation cell. The observations focused on the changes in mineralogy of the material and reflectance spectral changes during the oxidation process, and the ionic composition and chemical status of the solutions generated. Mineralogical investigations with hyperspectral and XRD analysis showed that frequent inundation produced extensive surface oxidation and a change from iron sulphide minerals to stable end members such as goethite. For the material experiencing moderate or occasional inundation, oxidation was less advanced and a different secondary mineral suite, with iron sulphate minerals such as jarosite, was present. Solutions generated from all the incubation cells were generally acidic with pH around 3.5, indicating that sulphide oxidation occurred rapidly in all cells. Sulphate concentrations in solutions from the different cells were calculated from the measurements of SO42− and other anions in solutions and may approximate the rates of sulphide oxidation. Potential environmental impacts were illustrated in terms of soil acidity, salinity and trace metal release, and the effects of the different wetting/drying regimes on the oxidative process also provided insight into potential weathering effects in a changing climate.

Distribution and partitioning of major and trace elements in pyrite-bearing sediments of a Mediterranean coastal lagoon

The formation of iron sulphide minerals exerts significant control on the behaviour of trace elements in sediments. In this study, three short sediment cores, retrieved from the remote Antinioti lagoon (N. Kerkyra Island, NW Greece), are investigated concerning the solid phase composition, distribution, and partitioning of major (Al, Fe) and trace elements (Cd, Cu, Mn, Pb, and Zn). According to 210Pb, the sediments sampled correspond to depositions of the last 120 years. The high amounts of organic carbon (4.1–27.5%) result in the formation of Fe sulphides, predominantly pyrite, already at the surface sediment layers. Pyrite morphologies include monocrystals, polyframboids, and complex FeS–FeS2 aggregates. According to synchrotron-generated micro X-ray fluorescence and X-ray absorption near-edge structure spectra, authigenically formed, Mn-containing, Fe(III) oxyhydroxides (goethite type) co-exist with pyrite in the sediments studied. Microscopic techniques evidence the formation of galena, sphalerite and CuS, whereas sequential extractions show that carbonates are important hosts for Mn, Cd, and Zn. However, significant percentages of non-lattice held elements are bound to Fe/Mn oxyhydroxides that resist reductive dissolution (on average 60% of Pb, 46% of Cd, 43% of Zn and 9% of Cu). The partitioning pattern changes drastically in the deeper part of the core that is influenced by freshwater inputs. In these sediments, the post-depositional pyritization mechanism, illustrated by overgrowths of Fe monosulphides on pre-existing pyrite grains, results in relatively high degree of pyritization that reaches 49% for Cd, 66% for Cu, 32% for Zn and 7% for Pb.

The Occurrence of Iron Sulfide Minerals and Their Indication Significance of Mineralization Process in the Panzhihua Fe‐Ti‐V Oxide Deposit, SW China

The Occurrence of Iron Sulfide Minerals and Their Indication Significance of Mineralization Process in the Panzhihua Fe‐Ti‐V Oxide Deposit, SW China

Early Cambrian high-temperature dolomite of the Rizu Series in the Jalal-Abad iron ore deposit, Central Iran

Early Cambrian dolomites associated with siltstone, volcanic, and volcaniclastic rocks are the host rocks of the Jalal-Abad iron deposit. Five types of dolomite are distinguished by petrographic studies; fine to medium crystalline, planar dolomites (Rd1, Rd2), medium to coarse crystalline (Rd3), cloudy, subhedral to euhedral dolomite cements (Cd1), and coarse to very coarse saddle dolomite cements (Cd2). The Rd1-type dolomites formed in replacement stages, while Rd2 and Rd3 dolomites are formed during the early diagenetic stage. Cd1- and Cd2-type dolomites were formed by recrystallization during hydrothermal processes. Cd1 and Cd2 dolomite is mainly associated with different types of iron oxide, quartz, and sulfide minerals. Investigations of fluid inclusions in saddle dolomites show homogenization temperatures from 160 to 260 °C, with a mean of 215 °C, and salinities between 32.1 and 38, with an average of 35.9 wt% NaCl equiv. The δ13C (PDB) compositions of the Rd1, Rd2, and Rd3 dolomites fall in the range of −5.30 to −2.97 and δ18O (PDB) values of dolomite vary between −9.54 and −8.58. The δ13C (PDB) compositions of Cd1 and Cd2 dolomites range from −11.00 to −4.17 and their δ18O (PDB) values from −16.80 to −8.89. The presence of volcanic and volcaniclastic rocks in the dolomitic sequences and crosscutting of dolomite sequences by intrusive bodies indicate magmatic activity as the heat source of high-temperature dolomitization. Based on the compiled data, we argue that high-temperature dolomitization was caused by hydrothermal fluids which migrated through faults and fractures during the Early Cambrian and, probably, later.

Surface passivation limited UO2 oxidative dissolution in the presence of FeS.

Iron sulfide minerals produced during in situ bioremediation of U can serve as an oxygen scavenger to retard uraninite (UO2) oxidation upon oxygen intrusion. Under persistent oxygen supply, however, iron sulfides become oxidized and depleted, giving rise to elevated dissolved oxygen (DO) levels and remobilization of U(IV). The present study investigated the mechanism that regulates UO2 oxidative dissolution rate in a flow-through system when oxygen breakthrough occurred as a function of mackinawite (FeS) and carbonate concentrations. The formation and evolution of surface layers on UO2 were characterized using XAS and XPS. During FeS inhibition period, the continuous supply of carbonate and calcium in the influent effectively complexed and removed oxidized U(VI) to preserve an intermediate U4O9 surface. When the FeS became depleted by oxidization, a transient, rapid dissolution of UO2 was observed along with DO breakthrough in the reactor. This rate was greater than during the preceding FeS inhibition period and control experiments in the absence of FeS. With increasing DO, the rate slowed and the rate-limiting step shifted from surface oxidation to U(VI) detachment as U(VI) passivation layers developed. In contrast, increasing the carbonate concentrations facilitated detachment of surface-associated U(VI) complexes and impeded the formation of U(VI) passivation layer. This study demonstrates the critical role of U(VI) surface layer formation versus U(VI) detachment in controlling UO2 oxidative dissolution rate during periods of variable oxygen presence under simulated groundwater conditions.

Factors Controlling In Situ Biogeochemical Transformation of Trichloroethene: Field Survey

AbstractWe have previously defined in situ biogeochemical transformation as the biogenic formation of reactive minerals that are capable of abiotically degrading chlorinated solvents such as trichloroethene without accumulation of degradation products such as vinyl chloride (AFCEE et al. ). This process has been implemented in biowalls used to intercept contaminated groundwater. Abiotic patterns of contaminant degradation were observed at Altus Air Force Base (AFB) and in an associated column study, but not at other sites including Dover AFB. These abiotic patterns were associated with biogenic formation of reactive iron sulfide minerals. Iron sulfides in the form of small individual grains, coatings on magnetite, and sulfur‐deficient pyrite framboids were observed in samples collected from the Altus AFB biowalls and one of the EPA columns. Larger iron sulfide grains coated with oxide layers were observed in samples collected from Dover AFB. Altus AFB and the EPA column differed from Dover AFB in that groundwater flow at Dover AFB was relatively slow and potentially reversing. High volumetric sulfate consumption rates, an abiotic pattern of trichloroethene (TCE) degradation, and the formation of small, high surface area iron sulfide particles were associated with relatively high rates of TCE removal via an abiotic pattern. Geochemical modeling demonstrated that iron monosulfides such as mackinawite were near saturation, and iron disulfides such as pyrite were supersaturated at all sites. This environmental condition can be supportive of nucleation of small particles rather than crystal growth leading to larger particles. When nucleation is dominant, small, high surface area, and reactive particles result. When crystal growth dominates the crystals are larger and have lower specific surface area and reactivity. These results taken together suggest that creation of a dynamic environment can promote biogeochemical transformation based on generation of reactive iron sulfides.

Acid Extraction Overestimates the Total Fe(II) in the Presence of Iron (Hydr)oxide and Sulfide Minerals

Acid extraction followed by a colorimetric assay is a widely used approach for the determination of the total Fe(II) in samples from suboxic and anoxic environments. Under sulfidogenic conditions, these samples can contain both metal sulfides and iron (hydr)oxides. Our comparisons between 0.5 N HCl extraction and X-ray absorption fine structure (XAFS) analysis of systems containing 40 mM Fe(III) as specific (hydr)oxides and 10 mM FeS showed that the presence of sulfide can result in significant overestimation of the total Fe(II) by acid extraction. The total Fe(II) determined by XAFS was consistent with the added FeS. However, the total Fe(II) determined by extraction was significantly higher and dependent on the nature of the Fe(III) (hydr)oxides; the rates and extent of Fe(II) production were higher with ferrihydrite and lepidocrocite than with goethite. Total Fe(II) concentrations determined following acid extraction might be overestimated when iron (hydr)oxides and sulfide minerals are present in samples.

Δ9-tetrahydrocannabinol does not substitute in rats trained to self-administer WIN55212-2

Iron sulfide minerals are abundant on Earth and are often treated as mine wastes. Thus, economically beneficial use of iron sulfides is of great significance, and one option is to use iron sulfide minerals to develop functional materials for wastewater treatment. In this study, nanostructured pyrrhotite (NPyr), which was manufactured by calcining natural pyrite, was used as the biofilm substratum in pyrrhotite autotrophic denitrification biofilters (PADBs). Two identical laboratory-scale biofilters (diameter, 10 mm; height, 50 cm) were set up. The HRT of the nanostructured PADBs was gradually reduced from 7.2 to 0.6 h over a 536-day trial. At a HRT of 1.2 h, average concentrations of N of 0.05 ± 0.01 mg/L and P of 0.03 ± 0.01 mg/L were achieved when treating real secondary effluent (N: 13.81 ± 1.52 mg/L, P: 2.44 ± 0.05 mg/L). The low concentrations of N and P achieved in the nanostructured PADB effluent at very short HRTs indicate the potential of this technology for tertiary wastewater treatment and in meeting strict discharge standards. High-throughput sequencing of 16S rRNA genes showed that Thiobacillus was the relatively most dominant genus (up to 87% relative abundance) in the PADBs. Transmission electron microscopy of the used NPyr indicated that P was mainly removed by the precipitation of FePO4(s). It was found that when treating the real wastewater, sulfur was oxidized to SO42− first due to the activity of Thiobacillus, and then SO42− was reduced to polysulfides (Sn2−) along the biofilters due to the activities of sulfate reducing bacteria. This observation highlights that the nanostructured PADB technology might be operated in a way of sustainable use of sulfur as an electron donor.

Computational Redox Potential Predictions: Applications to Inorganic and Organic Aqueous Complexes, and Complexes Adsorbed to Mineral Surfaces

Applications of redox processes range over a number of scientific fields. This review article summarizes the theory behind the calculation of redox potentials in solution for species such as organic compounds, inorganic complexes, actinides, battery materials, and mineral surface-bound-species. Different computational approaches to predict and determine redox potentials of electron transitions are discussed along with their respective pros and cons for the prediction of redox potentials. Subsequently, recommendations are made for certain necessary computational settings required for accurate calculation of redox potentials. This article reviews the importance of computational parameters, such as basis sets, density functional theory (DFT) functionals, and relativistic approaches and the role that physicochemical processes play on the shift of redox potentials, such as hydration or spin orbit coupling, and will aid in finding suitable combinations of approaches for different chemical and geochemical applications. Identifying cost-effective and credible computational approaches is essential to benchmark redox potential calculations against experiments. Once a good theoretical approach is found to model the chemistry and thermodynamics of the redox and electron transfer process, this knowledge can be incorporated into models of more complex reaction mechanisms that include diffusion in the solute, surface diffusion, and dehydration, to name a few. This knowledge is important to fully understand the nature of redox processes be it a geochemical process that dictates natural redox reactions or one that is being used for the optimization of a chemical process in industry. In addition, it will help identify materials that will be useful to design catalytic redox agents, to come up with materials to be used for batteries and photovoltaic processes, and to identify new and improved remediation strategies in environmental engineering, for example the reduction of actinides and their subsequent immobilization. Highly under-investigated is the role of redox-active semiconducting mineral surfaces as catalysts for promoting natural redox processes. Such knowledge is crucial to derive process-oriented mechanisms, kinetics, and rate laws for inorganic and organic redox processes in nature. In addition, molecular-level details still need to be explored and understood to plan for safer disposal of hazardous materials. In light of this, we include new research on the effect of iron-sulfide mineral surfaces, such as pyrite and mackinawite, on the redox chemistry of actinyl aqua complexes in aqueous solution.

Magnetite formation from ferrihydrite by hyperthermophilic archaea from Endeavour Segment, Juan de Fuca Ridge hydrothermal vent chimneys

Hyperthermophilic iron reducers are common in hydrothermal chimneys found along the Endeavour Segment in the northeastern Pacific Ocean based on culture-dependent estimates. However, information on the availability of Fe(III) (oxyhydr) oxides within these chimneys, the types of Fe(III) (oxyhydr) oxides utilized by the organisms, rates and environmental constraints of hyperthermophilic iron reduction, and mineral end products is needed to determine their biogeochemical significance and are addressed in this study. Thin-section petrography on the interior of a hydrothermal chimney from the Dante edifice at Endeavour showed a thin coat of Fe(III) (oxyhydr) oxide associated with amorphous silica on the exposed outer surfaces of pyrrhotite, sphalerite, and chalcopyrite in pore spaces, along with anhydrite precipitation in the pores that is indicative of seawater ingress. The iron sulfide minerals were likely oxidized to Fe(III) (oxyhydr) oxide with increasing pH and Eh due to cooling and seawater exposure, providing reactants for bioreduction. Culture-dependent estimates of hyperthermophilic iron reducer abundances in this sample were 1740 and 10 cells per gram (dry weight) of material from the outer surface and the marcasite-sphalerite-rich interior, respectively. Two hyperthermophilic iron reducers, Hyperthermus sp. Ro04 and Pyrodictium sp. Su06, were isolated from other active hydrothermal chimneys on the Endeavour Segment. Strain Ro04 is a neutrophilic (pH opt 7-8) heterotroph, while strain Su06 is a mildly acidophilic (pH opt 5), hydrogenotrophic autotroph, both with optimal growth temperatures of 90-92 °C. Mössbauer spectroscopy of the iron oxides before and after growth demonstrated that both organisms form nanophase (<12 nm) magnetite [Fe3 O4 ] from laboratory-synthesized ferrihydrite [Fe10 O14 (OH)2 ] with no detectable mineral intermediates. They produced up to 40 mm Fe(2+) in a growth-dependent manner, while all abiotic and biotic controls produced <3 mm Fe(2+) . Hyperthermophilic iron reducers may have a growth advantage over other hyperthermophiles in hydrothermal systems that are mildly acidic where mineral weathering at increased temperatures occurs.

Field application of biogeochemical reductive dechlorination by permeable reactive barrier

Biogeochemical reductive dechlorination (BiRD) uses biotically generated iron sulphide minerals to abiotically reduce chlorinated aliphatic hydrocarbons (CAH). BiRD was applied by permeable reactive barrier (PRB) at Dover Air Force Base, the first time this approach was tested in the field. Iron sulphide minerals were generated in trenches filled with iron rich sand (iron), mulch (organic), mineral gypsum (sulphate), and limestone (pH control). For comparison purposes, trenches were also filled with sand, mulch, and limestone to create conditions favourable to biostimulation. Compared to biostimulation, BiRD showed little evidence of methanogenesis, indicating that the reductive capacity of the applied organic was converted to mineral iron sulphide rather than methane. CAH treatment in the biostimulation PRB appeared to be incomplete after 150 days, apparently stalling at DCE. In comparison, for the BiRD treatment area, CAH treatment appeared to be rapid and reduced PCE, TCE and DCE levels within the 150 day time period.

The microbiology of acid sulfate soils and sulfidic sediments

Acid sulfate soils and their associated sulfidic sediments present a major hazard to sustainable farming, water security and urban infrastructure. Traditionally these soils are limed in order to neutralise the ‘leachate’ that is a public healthhazardand toxic toaquaticorganisms. Itmaybemore sustainable to exploit the soil microorganisms capable of sequesteringmetals to remediate these soils. Until recently, little was known about the microbial ecology of these environments. The soils have a moderately acidic (pH 4) chemistry and a unique ecosystem where the microbial community composition is correlated to bioaccessible metal concentrations. These environments have the potential to provide novel insights into how environmental conditions shape the microbiome that can be exploited for biotechnologies. The formation and management of acid sulfate soils Worldwide, the pressure for land to meet food security and urban development needs can lead to the disturbance of waterlogged soils containing iron-sulfide minerals. These minerals are oxidised to form acid sulfate soils (Figure 1); a toxic legacy of rain-induced acidic and metalliferous groundwaters that lead to the corrosion of infrastructure and the severe deterioration of water quality. The associated formation of monosulfides in affected waterways is equally problematic, capable of de-oxygenating the aquatic ecosystem in seconds if disturbed.

Nano-FeS Inhibits UO2 Reoxidation under Varied Oxic Conditions

Bioreductive in situ treatment of U-contaminated groundwater can convert soluble U(VI) species to immobile reduced U(IV) solid phases such as UO2(s) to contain U movement. Once active bioremediation is halted, UO2 may be subsequently reoxidized if oxidants such as oxygen enter the reducing zone. However, iron sulfide minerals that form during bioreduction may serve as electron sources or oxygen scavengers and inhibit UO2 reoxidation upon oxygen intrusion. In this study, flow-through reactor experiments examined the abiotic kinetics of UO2 oxidative dissolution in the presence of oxygen and nanoparticulate FeS as a function of pH, dissolved oxygen (DO) concentration, and FeS content. The UO2 dissolution rates in the presence of FeS were over 1 order of magnitude lower than those in the absence of FeS under otherwise comparable oxic conditions. FeS effectively scavenged DO and preferentially reacted with oxygen, contributing to a largely unreacted UO2 solid phase during an "inhibition period" as determined by X-ray absorption spectroscopy (XAS). The removal of DO by FeS was significant but incomplete during the inhibition period, resulting in surface-oxidation-limited dissolution and greater UO2 dissolution rate with increasing influent DO concentration and decreasing FeS content. Although the rate was independent of solution pH in the range of 6.1-8.1, the length of the inhibition period was shortened by substantial FeS dissolution at slightly acidic pH. The reducing capacity of FeS was greatest at basic pH where surface-mediated FeS oxidation dominated.

Molecular assays advance understanding of sulfate reduction despite cryptic cycles

Abiotic and secondary biotic reactions can contribute to the formation of cryptic biogeochemical cycles, resulting in an underestimation of carbon and nutrient budgets. This Texas coastal estuary sediment study provided a unique opportunity to use multidisciplinary RNA-based molecular and geochemical approaches to identify cryptic cycles associated with sulfate reduction, a commonly measured biogeochemical process considered to be the predominant anoxic terminal electron accepting process in shallow marine environments. Active sulfate reduction within an environment is typically determined by the detection of sulfides. However, a biologically driven cryptic cycle was determined by identifying metabolically active sulfate reducing and sulfur oxidizing lineages co-locating within the sediments, effectively masking sulfide production through re-oxidation back to sulfate. Similar co-location of sulfate and iron reducing lineages prevented the detection of sulfides through the formation of iron sulfide minerals, producing a geochemically driven cryptic cycle. We also showed that sulfate reduction rates determined by 35SO42− incubation analysis can be positively correlated with dsrA transcript abundance, and thus this molecular technique may be a proxy for the prohibitive radioactive method. Based on these results, support is given to a synergistic geochemical and molecular biological strategy to better provide an understanding of marine sulfur cycle.

Formation of hydrogen peroxide by chalcopyrite and its influence on flotation

Formation of hydrogen peroxide (H2O2), an oxidizing agent stronger than oxygen, by chalcopyrite (CuFeS2), which is a copper iron sulfide mineral, during grinding, was investigated. It was observed that chalcopyrite generated H2O2 in pulp liquid during wet grinding and also in the solids when placed in water immediately after dry grinding. The generation of H2O2 during either wet or dry grinding was thought to be due to a reaction between chalcopyrite and water where the mineral surface is catalytically active in producing ●OH free radicals by breaking down the water molecule. The effect of pH in the grinding medium or in the water in which solids are added immediately after dry grinding showed that the lower the pH value, the higher the H2O2 generation. When chalcopyrite and pyrite are mixed in different proportions, the formation of H2O2 was seen to increase with increasing pyrite fraction in the mixed composition. The results of H2O2 formation in pulp liquid of chalcopyrite and together with pyrite at different experimental conditions have been explained by Eh-pH diagrams of these minerals. This study highlights the necessity of revisiting the electrochemical and/or galvanic interaction mechanisms between the chalcopyrite and pyrite in terms of their flotation behavior.

Geochemical behavior of arsenic in reducing sulfidic sediments of reservoir contaminated by acid mine drainage

The sediment from an acid mine drainage affected reservoir of Guizhou province of China has the iron and arsenic concentration of about 400 and 2.6 g/kg, respectively. Sediment cores were collected, and were used to study the arsenic behavior in the seriously acidified reservoir from the viewpoint of chemical thermodynamics. The limestone neutralization and ferric iron hydrolysis regulated the porewater pH from about 2.9–5.8. The reductive dissolution of As–Fe-rich (hydr)oxides under the mild acidic conditions was the main mechanism for the release of absorbed arsenic into porewater. The maximum concentrations of iron, sulfate and arsenic reached to about 2,800, 9,000 and 1 mg/l, respectively. Arsenic speciation transformation and hydrous ferric oxide (HFO) crystallization enhanced the arsenic mobility in sediment. In addition, the iron sulfide minerals diagenesis could play a role in removing the dissolved arsenic from porewater. The actual distribution of arsenic concentration in porewater was well simulated using the model of surface complexation of arsenic to HFO. Although arsenic concentration in porewater could be above 100 times higher than that of reservoir water, it was not easy to release into the reservoir water through diffusion, because the shallow sediment had relatively strong arsenic adsorption capacity, and new HFO could be generated continuously at the sediment water interface.

The remarkable longevity of submarine plumes: Implications for the hydrothermal input of iron to the deep-ocean

The longevity of submarine plumes generated at sea-floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep-ocean. Decades of observations of episodic “event plumes” suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering. Here, we use analog experiments on turbulent, hot particle-laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability. Where hydrothermal clouds are enriched in fine minerals, this “particle diffusive convection” can extend the longevity of an event plume to 2 yr after its emplacement. The very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals. We develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron-sulfide minerals settling through the cloud by diffusive convection. A key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to 19±3 nM, which represents about 5% of the global hydrothermal discharge. If these results are representative of all hydrothermal vent fields, hydrothermal systems could provide 75% of the global budget of dissolved iron in the deep-ocean. Regionally, this flux is expected to scale in magnitude with mid-ocean ridge heat flow, consistent with observations and global ocean models.

Open-pit coal-mining effects on rice paddy soil composition and metal bioavailability to Oryza sativa L. plants in Cam Pha, northeastern Vietnam

This study quantified Cd, Pb, and Cu content, and the soil-plant transfer factors of these elements in rice paddies within Cam Pha, Quang Ninh province, northeastern Vietnam. The rice paddies are located at a distance of 2 km from the large Coc Sau open-pit coal mine. Electron microprobe analysis combined with backscattered electron imaging and energy-dispersive spectroscopy revealed a relatively high proportion of carbon particles rimmed by an iron sulfide mineral (probably pyrite) in the quartz-clay matrix of rice paddy soils at 20-30 cm depth. Bulk chemical analysis of these soils revealed the presence of Cd, Cu, and Pb at concentrations of 0.146±0.004, 23.3±0.1, and 23.5±0.1 mg/kg which exceeded calculated background concentrations of 0.006±0.004, 1.9±0.5, and 2.4±1.5 mg/kg respectively at one of the sites. Metals and metalloids in Cam Pha rice paddy soils, including As, Cd, Cr, Cu, Hg, Mn, Ni, Pb, and Zn, were found in concentrations ranging from 0.2±0.1 to 140±3 mg/kg, which were in close agreement with toxic metal contents in mine tailings and Coc Sau coal samples, suggesting mining operations as a major cause of paddy soil contamination. Native and model Oryza sativa L. rice plants were grown in the laboratory in a growth medium to which up to 1.5 mg/kg of paddy soil from Cam Pha was added to investigate the effects on plant growth. A decrease in growth by up to 60% with respect to a control sample was found for model plants, whereas a decrease of only 10% was observed for native (Nep cai hoa vang variety) rice plants. This result suggests an adaptation of native Cam Pha rice plants to toxic metals in the agricultural lands. The Cd, Cu, and Pb contents of the native rice plants from Cam Pha paddies exceeded permitted levels in foods. Cadmium and Pb were highest in the rice plant roots with concentrations of 0.84±0.02 and 7.7±0.3 mg/kg, suggesting an intake of these metals into the rice plant as shown, for example, by Cd and Pb concentrations of 0.09±0.01 and 0.10±0.04 mg/kg respectively in the rice grain endosperm. The adaptation of native rice plants, combined with bioaccumulation ratios of 1±0.6 to 1.4±0.7 calculated for Cd transfer to the rice grain endosperm, and maximum Cd transfer factors of 4.3±2.1 to the plant roots, strongly suggest a continuous input of some toxic metals from coal-mining operations to agricultural lands in the region of Cam Pha. In addition, our results imply a sustained absorption of metals by native rice plant varieties, which may lead to metal accumulation (e.g., Cd) in human organs and in turn to severe disease.

Arsenic Bioremediation by Biogenic Iron Oxides and Sulfides

Microcosms containing sediment from an aquifer in Cambodia with naturally elevated levels of arsenic in the associated groundwater were used to evaluate the effectiveness of microbially mediated production of iron minerals for in situ As remediation. The microcosms were first incubated without amendments for 28 days, and the release of As and other geogenic chemicals from the sediments into the aqueous phase was monitored. Nitrate or a mixture of sulfate and lactate was then added to stimulate biological Fe(II) oxidation or sulfate reduction, respectively. Without treatment, soluble As concentrations reached 3.9 ± 0.9 μM at the end of the 143-day experiment. However, in the nitrate- and sulfate-plus-lactate-amended microcosms, soluble As levels decreased to 0.01 and 0.41 ± 0.13 μM, respectively, by the end of the experiment. Analyses using a range of biogeochemical and mineralogical tools indicated that sorption onto freshly formed hydrous ferric oxide (HFO) and iron sulfide mineral phases are the likely mechanisms for As removal in the respective treatments. Incorporation of the experimental results into a one-dimensional transport-reaction model suggests that, under conditions representative of the Cambodian aquifer, the in situ precipitation of HFO would be effective in bringing groundwater into compliance with the World Health Organization (WHO) provisional guideline value for As (10 ppb or 0.13 μM), although soluble Mn release accompanying microbial Fe(II) oxidation presents a potential health concern. In contrast, production of biogenic iron sulfide minerals would not remediate the groundwater As concentration below the recommended WHO limit.

Deposition of Biogenic Iron Minerals in a Methane Oxidizing Microbial Mat

The syntrophic community between anaerobic methanotrophic archaea and sulfate reducing bacteria forms thick, black layers within multi-layered microbial mats in chimney-like carbonate concretions of methane seeps located in the Black Sea Crimean shelf. The microbial consortium conducts anaerobic oxidation of methane, which leads to the formation of mainly two biomineral by-products, calcium carbonates and iron sulfides, building up these chimneys. Iron sulfides are generated by the microbial reduction of oxidized sulfur compounds in the microbial mats. Here we show that sulfate reducing bacteria deposit biogenic iron sulfides extra- and intracellularly, the latter in magnetosome-like chains. These chains appear to be stable after cell lysis and tend to attach to cell debris within the microbial mat. The particles may be important nuclei for larger iron sulfide mineral aggregates.